Manufacturers almost always feel pressure to lower costs, but in connection with cellular basestations that pressure, along with a desire to reduce weight and volume, is particularly strong. Lower basestation costs allow a basestation manufacturer to more effectively compete, and they also lead to better and/or less expensive cellular telephony services for ultimate consumers. But in connection with cellular basestations, the benefits of lower costs are amplified. Lower basestation costs, along with reduced basestation weight and volume, allow a cellular-service provider to install more basestations in a given area, such as a city. This not only provides better cellular coverage, but it allows the basestations and mobile stations with which they communicate to transmit at lower power levels because distances between the two are usually smaller (e.g., link propagation losses increase approximately as the fourth-power of link distance). The use of lower power levels in mobile stations leads to the use of smaller batteries and correspondingly smaller cell phones. And, the use of lower power leads to smaller cells and greater reuse of a licensed frequency band through which cellular service is being provided. Greater frequency reuse in a given area leads to the conveyance of more communications in the given area using the same spectrum. Thus, lower basestation costs have far reaching consequences beyond the direct economic advantages achieved.
The goal of reducing cellular basestation costs faces many obstacles. One of these obstacles is the diverse number of cellular frequency bands and standards in use worldwide. Moreover, new standards are being repeatedly proposed for the provision of newer and better cellular services in the future. The following table illustrates many of these:
TABLE ISURVEY OF EXISTING AND PROPOSED CELLULAR SYSTEMSCellular BandDnlink (MHz)Uplink (MHz)450 MHz462.5-467.5452-5-457.5US CELLULAR (AMPS)869-894824-849SMR, iDEN851-869806-824GSM-900935-960890-915EGSM925-960880-915DCS-1800 (GSM-Europe)1805-18801710-1785CDMA Korea1840-18701750-17801900 (US PCS)1930-19901850-1910UMTS/IMT-20002110-21701920-1980Each different frequency band and bandwidth imposes somewhat different constraints on the basestation hardware. Regardless of band and bandwidth considerations, different standards call out entirely different modulation techniques that impose still other constraints on the basestation hardware.
The modulation techniques fall into two different families, distinguished from one another by bandwidth. One is the wide bandwidth family, which transmits either a spread-spectrum signal using code division multiple access (CDMA) techniques or an orthogonal frequency-division multiplex (OFDM) signal. The other is the narrow bandwidth family which transmits a narrow bandwidth signal using either binary or 8-ary Gaussian minimum shift keying (GMSK) and similar techniques. The narrow bandwidth family of modulation techniques is a characteristic of Global System for Mobile communications (GSM) systems. For the purposes of the present invention, narrowband (NB) signals and the narrow bandwidth family shall be characterized as using a bandwidth of less than 750 KHz per channel while wideband (WB) signals and the wide bandwidth family shall be characterized as using a bandwidth of greater than 750 KHz. With all these diverse systems and proposed systems, economies of scale are difficult to achieve, and when one system is adopted, flexibility in making future changes, upgrades, and expansion is often sacrificed. This results in severe economic inefficiency that leads to higher basestation costs.
Attempts are ongoing to define basestation standards that will address these economic realities in an attempt to commoditize basestation components, and hopefully lead to lower costs. One such proposal is the Open Base Station Architecture Initiative (OBSAI) and another is the more narrowly focused Common Radio Protocol Initiative (CPRI). These initiatives contemplate the use of a somewhat generic RF module that can accommodate at least two diverse frequency bands and can accommodate multiple adjacent channels that may be placed anywhere in the supported frequency bands. These initiatives reflect a common belief that a single RF module that accommodates a greater number of frequency bands or accommodates more than one family of modulation techniques will be economically inefficient or otherwise lead to larger and/or heavier basestations.
But RF modules that accommodate only one family of modulation techniques are economically inefficient by promoting high economic switching costs. Such an approach constrains future expansion, upgrades, and change options. In short, after purchasing hardware that supports only one family of modulation techniques, then the overall costs of all future expansion, upgrading, and changes will almost certainly be higher in the non-selected family of modulation techniques, even if otherwise technically and economically superior, because of high switching costs associated with changing between families of modulation techniques.
Typically, digital signal processing is available at a lower cost than that of corresponding analog processing. Thus, in order to lower basestation costs, RF module designs for either wide or narrow bandwidth modulation techniques attempt to process communication signals using digital techniques as much as practical. But the wide bandwidth family of modulation techniques presents particular challenges with respect to the wide bandwidth that must be accommodated to process a multiple channel CDMA signal. These challenges are exacerbated when higher harmonics of the multichannel baseband communication signal are to be processed to accommodate linearization of a high power amplifier. Although a high sample rate is demanded, the wide bandwidth family of modulation techniques lends itself to a great amount of digital processing to offset costs associated with the high sample rates demanded. Even direct upconversion, where image and LO leakage signal components fall in-band, is practical for multichannel CDMA signals because the specifications for adjacent channel power ratio (ACPR) are relatively modest. But tuning or adjusting operations to achieve optimal conditions may be required before even this modest specification can be met. Special tuning operations are undesirable because they increase manufacturing costs as well as ongoing costs due to an increased risk of becoming detuned in response to the ravages of time, temperature cycling, and jostling.
On the other hand, the narrow bandwidth family of modulation techniques presents particular challenges with respect to severe ACPR specifications. Typically, the narrow bandwidth family of modulation techniques results in more modest bandwidth requirements, even for a multichannel communication signal that accommodates higher harmonics for high power amplifier linearization. But the severe ACPR specifications typically results in an inability to use direct upconversion for a multichannel signal because image and LO leakage signal components need to be separated in frequency from the transmit band so that they may be filtered off to satisfy the severe ACPR specifications.
In accordance with conventional techniques, in order for an RF module to accommodate both families of modulations, the RF module will need to include the higher cost components otherwise unique to each family to meet the particular challenges of both families, resulting in a particularly expensive item that is likely to be larger and heavier.